Quelle: Von Kmusser – Eigenes Werk, Elevation data from SRTM, hydrologic data from the National Hydrography Dataset, urban areas from Vector Map, all other features from the National Atlas., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=12520461

Subsidence along the Atlantic Coast of North America: Insights from GPS and late Holocene relative sea level data
The Atlantic Coast of North America is increasingly affected by flooding associated with tropical and extratropical storms, exacerbated by the combined effects of accelerated sea-level rise and land subsidence. The region includes the collapsing forebulge of the Laurentide Ice Sheet. High-quality records of late Holocene relative sea-level (RSL) rise are now available, allowing separation of long-term glacial isostatic adjustment-induced displacement from modern vertical displacement measured by GPS. We compare geological records of late Holocene RSL to present-day vertical rates from GPS. For many coastal areas there is no significant difference between these independent data. Exceptions occur in areas of recent excessive groundwater extraction, between Virginia (38°N) and South Carolina (32.5°N). The present-day subsidence rates in these areas are approximately double the long-term geologic rates, which has important implications for flood mitigation.Tide gauge records, therefore, should be used with caution for studying sea-level rise in this region.

Past and present sea levels in the Chesapeake Bay Region, USA:What this means for the future

Scientists write that sea-level rise (3.4 mm/yr) is faster in the Chesapeake Bay region than any other location on the Atlantic coast of North America, and twice the global average (1.7 mm/yr). They have found that dated interglacial deposits suggest that relative sea levels in the Chesapeake Bay region deviate from global trends over a range of timescales.

In a new article for GSA Today, authors Benjamin DeJong and colleagues write that sea-level rise (3.4 mm/yr) is faster in the Chesapeake Bay region than any other location on the Atlantic coast of North America, and twice the global average (1.7 mm/yr). They have found that dated interglacial deposits suggest that relative sea levels in the Chesapeake Bay region deviate from global trends over a range of timescales.

According to DeJong and colleagues, “Glacio-isostatic adjustment of the land surface from loading and unloading of continental ice is likely responsible for these deviations, but our understanding of the scale and timeframe over which isostatic response operates in this region remains incomplete because dated sea-level proxies are mostly limited to the Holocene and to deposits 80 ka or older.” To better understand glacio-isostatic control over past and present relative sea level, DeJong and colleagues applied a suite of dating methods to the stratigraphy of the Blackwater National Wildlife Refuge, one of the most rapidly subsiding and lowest-elevation surfaces bordering Chesapeake Bay. Their data indicate that the region was submerged at least for portions of marine isotope stage (MIS) 3 (about 30 to 60 thousand years ago), although, they note, multiple proxies suggest that global sea level was 40 to 80 meters lower than today.

Today, MIS 3 deposits are above sea level because they were raised by the Last Glacial Maximum forebulge, but decay of that same forebulge is causing ongoing subsidence. “These results,” they write, “suggest that glacio-isostasy controlled relative sea level in the mid-Atlantic region for tens of thousands of years following retreat of the Laurentide Ice Sheet and continues to influence relative sea level in the region.” The study finds that isostatically driven subsidence of the Chesapeake Bay region will continue for millennia, exacerbating the effects of global sea-level rise and impacting the region’s large population centers and valuable coastal natural resources.

New research confirms that the land under the Chesapeake Bay is sinking rapidly and projects that Washington, D.C., could drop by six or more inches in the next century–adding to the problems of sea-level rise.

This falling land will exacerbate the flooding that the nation’s capital faces from rising ocean waters due to a warming climate and melting ice sheets–accelerating the threat to the region’s monuments, roads, wildlife refuges, and military installations.

For sixty years, tide gauges have shown that sea level in the Chesapeake is rising at twice the global average rate and faster than elsewhere on the East Coast. And geologists have hypothesized for several decades that land in this area, pushed up by the weight of a pre-historic ice sheet to the north, has been settling back down since the ice melted.

The new study–based on extensive drilling in the coastal plain of Maryland–confirms this hypothesis, and provides a firm estimate of how quickly this drop is happening. Additionally, the researchers’ detailed field data make clear that the land sinking around Washington is not primarily driven by human influence, such as groundwater withdrawals, but instead is a long-term geological process that will continue unabated for tens of thousands of years, independent from human land use or climate change.

The new research was conducted by a team of geologists from the University of Vermont, the U.S. Geological Survey, and other institutions. The results were presented online July 27 in the journal GSA Today.

Geological Waterbed

Washington’s woes come from what geologists call “forebulge collapse.” During the last ice age, a mile-high North American ice sheet, that stretched as far south as Long Island, N.Y., piled so much weight on the Earth that underlying mantle rock flowed slowly outward, away from the ice. In response, the land surface to the south, under the Chesapeake Bay region, bulged up. Then, about 20,000 years ago, the ice sheet began melting away, allowing the forebulge to sink again.

“It’s a bit like sitting on one side of a water bed filled with very thick honey,” explains Ben DeJong, the lead author on the new study, who conducted the research as a doctoral student at UVM’s Rubenstein School of Environment and Natural Resources with support from the U.S. Geological Survey, “then the other side goes up. But when you stand, the bulge comes down again.”

The new research provides the first high-resolution data from the same latitude as Washington, D.C., DeJong said, of how this forebulge has subsided–and will continue to. “Until recently, the age of the thing was really poorly constrained,” he said.

To design the study, DeJong and others drilled seventy boreholes, many up to a hundred feet deep, in and around the Blackwater National Wildlife Refuge, near Washington, on the Chesapeake’s eastern shore. Then he examined layers of sediment in these deep cores, using a suite of techniques to calculate the age of the sand, other rocks, and organic matter in each layer.

Combining this data with high-resolution LiDAR and GPS map data allowed the team–that included scientists from UVM, the US Geological Survey, Utah State University, Berkeley Geochronology Center, and Imperial College, London–to create a detailed 3D portrait of both the current and previous post-glacial geological periods in the Chesapeake, stretching back several million years. This longer view gives the geologists confidence that they have a “bullet-proof” model, DeJong says, showing that the region today is early in a period of land subsidence that will last for millennia.

Wet Feet

“Right now is the time to start making preparations,” said DeJong. “Six extra inches of water really matters in this part of the world,” he says–adding urgency to the models of the Intergovernmental Panel on Climate Change that project roughly one to three or more feet of global sea-level rise by 2100 from global warming.

“It’s ironic that the nation’s capital–the place least responsive to the dangers of climate change–is sitting in one of the worst spots it could be in terms of this land subsidence,” said Paul Bierman, a UVM geologist and the senior author on the new paper. “Will the Congress just sit there with their feet getting ever wetter? What’s next, forebulge denial?”

Late Holocene sea level variability and Atlantic Meridional Overturning CirculationPre-twentieth century sea level (SL) variability remains poorly understood due to limits of tide gauge records, low temporal resolution of tidal marsh records, and regional anomalies caused by dynamic ocean processes, notably multidecadal changes in Atlantic Meridional Overturning Circulation (AMOC). We examined SL and AMOC variability along the eastern United States over the last 2000 years, using a SL curve constructed from proxy sea surface temperature (SST) records from Chesapeake Bay, and twentieth century SL-sea surface temperature (SST) relations derived from tide gauges and instrumental SST. The SL curve shows multidecadal-scale variability (20–30 years) during the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA), as well as the twentieth century. During these SL oscillations, short-term rates ranged from 2 to 4 mm yr−1, roughly similar to those of the last few decades. These oscillations likely represent internal modes of climate variability related to AMOC variability and originating at high latitudes, although the exact mechanisms remain unclear. Results imply that dynamic ocean changes, in addition to thermosteric, glacio-eustatic, or glacio-isostatic processes are an inherent part of SL variability in coastal regions, even during millennial-scale climate oscillations such as the MCA and LIA and should be factored into efforts that use tide gauges and tidal marsh sediments to understand global sea level rise.